This invention relates generally to the use and storage of hydrogen gas.
The use of hydrogen for a variety of applications has received a great deal of attention in recent years. For example, hydrogen has been positioned as an alternative to fossil fuels for energy, and for the operation of internal combustion engines in vehicles. Hydrogen can be combined with oxygen via combustion, and via oxidation/reduction reactions in a fuel cell device. Hydrogen-based fuel cells have now become a viable source of energy, with numerous advantages over petroleum-based engines. In general, the fuel cells are more efficient and quiet; operate at lower temperatures; operate with less friction; and are less polluting. As a fuel, hydrogen provides a number of advantages. For example, the gas is very abundant, clean, affordable, and renewable. The primary product of the hydrogen reactions—water—is non-polluting, and can be recycled to regenerate hydrogen and oxygen.
An important factor which can limit the widespread utilization of hydrogen is related to its storage and transport. Hydrogen is flammable over a wide range of concentrations in air, and at low spark temperatures. Thus, the storage and distribution of the gas is highly regulated. Frequently, hydrogen gas is stored at high pressure in a gas cylinder, e.g., a steel or composite tank. The walls of the cylinder need to be quite thick. This requirement makes the container heavy, and difficult to store and transport.
Hydrogen can also be stored in the liquid phase. In fact, hydrogen storage in liquid form can sometimes be more efficient than storage in the gas phase. However, very high-purity hydrogen is usually required. Moreover, since liquification temperatures can be as low as about −253° C., a special container capable of withstanding cryogenic temperatures is usually necessary. These requirements detract from the economic viability of liquid-phase storage.
Hydrogen can also be stored in the form of a metallic compound. For example, a variety of metals—alkali, alkaline earth, boron, aluminum, and the like—combine with hydrogen to form metal hydrides (usually in particulate form). When hydrogen is needed for a specific purpose, the metal hydride compounds can be heated to liberate the hydrogen.
While the storage of hydrogen as a metal alloy is useful in some applications, there are disadvantages as well. For example, the metal component is heavy, adding to the difficulty in transporting the material. Furthermore, the temperature needed to liberate the hydrogen from the metal can be quite high, e.g., greater than about 300° C. Moreover, storage compounds such as sodium hydride are caustic, flammable, and capable of violent reaction with water.
The storage of hydrogen in nanotubes is also being investigated. For example, hydrogen could be incorporated into porous carbon nanotubes. While further work will probably be undertaken on this concept, some of the apparent drawbacks at this stage of development are significant. For example, the nanotubes can be difficult to manufacture. Moreover, they may not be able to withstand the higher gas pressures required for large-scale hydrogen storage.
The use of hollow spheres to store hydrogen has also been studied. For example, a brief description of using glass “microballoons” is provided by I. Lewkowicz, in “Spherical Hydrogen Targets for Laser-Produced Fusion”, J. Phys. D: Appl. Phys. Vol. 7, 1974. The article discusses the possibility of using stable glass microballoons as hydrogen containers, and introducing hydrogen into the microballoons by permeation under high pressure. Moreover, Hearley et al discuss the possibility of hydrogen storage in glass microspheres (U.S. Patent Application Publication 2004/0213998 A1). Commercially-available glass spheres have walls which are permeable to hydrogen when they are heated. The spheres are charged with hydrogen by heating them in a high-pressure environment to cause the gas to permeate the walls and migrate into the interior. Once filled, the spheres are cooled, to lock the hydrogen inside. When the hydrogen is needed for a particular end use, the spheres can be re-heated, allowing the gas to permeate out of the hollow interior. The Hearley publication also discusses other potential hydrogen containers, e.g., various microparticles, hollow polymeric microspheres, and metal hydride materials.
The use of glass microspheres to selectively store and release hydrogen is a promising concept, and certainly worthy of additional development. However, there are some disadvantages involved in using glass microspheres. For example, the formation of hollow glass microspheres can be somewhat energy-intensive, because of their relatively high melting point. The microspheres are typically fabricated in a high-temperature drop tower, which can require very precise conditions, e.g., precise temperature and flow control. Moreover, the glass spheres generally exhibit low permeability to hydrogen, which limits the rate at which hydrogen can be infused into the spheres and then released by way of permeation. This drawback can be addressed to some degree by carrying out the permeation at relatively high temperatures, or by releasing the hydrogen in a mechanical manner, e.g., by crushing the spheres. However, the high temperatures clearly result in higher energy costs. Moreover, the destruction of the spheres prevents their re-use, and raises disposal issues as well.
In view of the preceding discussion, it should be apparent that new methods for storing and transporting hydrogen would be of great interest. The methods should be capable of securely storing hydrogen under considerably high pressure, and then releasing the hydrogen upon demand. These processes should also employ a relatively inexpensive storage medium which can be readily adjusted to initiate the flow of hydrogen, or to shut off such flow. The storage medium should also be relatively lightweight, to allow economical transport of the hydrogen. Moreover, the storage process should be compatible with the equipment which makes use of the released hydrogen, e.g., fuel cells. It would also be of considerable interest if the process could be used repeatedly, e.g., employing a storage medium which could be continuously recycled for additional use.